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Scientists have shown that the outbursts of Eta Carinae, the Milky Way’s biggest, brightest and perhaps most studied star after the Sun, could be driven by an entirely new type of stellar explosion that is fainter than a typical supernova.

Eta Carinae is surrounded by the Homunculus Nebula, a cloud of gas and dust thrown off the star in a brightening event that was observed from Earth in 1843. The resulting blast wave was found to be less energetic than a typical supernova event, meaning that Eta Carinae could be related to a class of faint stellar explosions also observed in other galaxies.

"There is a class of stellar explosions going off in other galaxies for which we still don't know the cause, but Eta Carinae is the prototype," says Nathan Smith of the University of California, Berkeley.

This sequence of images show's an artist's conception of the expanding blast wave from Eta Carinae's 1843 eruption. The first image shows the star as it may have appeared before the eruption, as a hot blue supergiant star surrounded by an older shell of gas that was ejected in a previous outburst about 1,000 years ago. Then in 1843, Eta Carinae suffered its explosive giant outburst, which created the well-known two-lobed Homunculus nebula, plus a fast shock wave propagating ahead of the Homunculus. As time proceeds, both the faster shock wave and the denser Homunculus nebula expand and fill the interior of the old shell. Eventually, the faster blast wave begins to catch-up with and overtake parts of the older shell, producing a bright fireworks display that heats the older shell. Images: Gemini Observatory, artwork by Lynette Cook.

As well as the Homunculus nebula, a faint shell of debris from an even earlier explosion that occurred around 1,000 years ago is also visible. Presumably blown off by the star's fierce wind, the shells of gas and dust are moving slowly – at speeds of 650 kilometres per second or less – compared to the blast shell of a supernova.

Using the international Gemini South 8-metre telescope and the Blanco 4-metre telescope at Cerro Tololo Inter-American Observatory in Chile, Smith and colleagues studied the enigmatic star and noticed something new: extremely fast filaments of gas speeding away from the star at five times the speed of the debris in the Homunculus nebula.

“The new observations show very fast material – much faster than we have seen previously – that originated in the same 1843 event that ejected the slower Homunculus,” Smith tells Astronomy Now. “The speeds are closer to speeds one normally sees in a supernova, but that doesn't mean that this 1843 event was a supernova. In fact, it wasn't, but it does seem to have been an explosion that behaved similar to a supernova but with less total energy.”

Smith says that “the cool part” is that the faster material from the 1843 event has now caught up with and is crashing into the bigger older shell, generating X-rays that have been observed by the orbiting Chandra Observatory. The new observations are forcing astronomers to modify their interpretation of what happened in the 1843 eruption.

"Rather than a steady wind blowing off the outer layers, it seems to have been an explosion that started deep inside the star and blasted off its outer layers,” says Smith. “It takes a new mechanism to cause explosions like this."

If Smith's interpretation is correct, supermassive stars like Eta
Carinae, which probably once had the same mass as 150 Suns, may blow off large amounts of mass in periodic explosions as they approach the end of their lives, before meeting their final demise in a cataclysmic supernova explosion that blows the star to smithereens and leaves behind a black hole.

“Eta Carinae is giving us clues that there are apparently precursor explosions that happen well before the real supernova,” Smith explains. “These precursor explosions evidently don't destroy the star completely, and it looks like they can happen multiple times. Exactly how many times they happen before the supernova, how much time passes between outbursts, and the number of future explosions that Eta Carinae will have before it really dies are questions that we still don't have very confident answers to.”

The answers to these questions will have important implications for our understanding of how massive stars end their lives. Such large stars burn brightly for only a few million years, all the while shedding mass as the intense light pushes the outer layers of the star away in a stellar wind. After 2 to 3 million years of such activity, Eta Carinae now weighs about 90 to 100 solar masses, having shed about 10 solar masses in its most recent 1843 eruption alone.

"These explosions may be the primary way by which massive stars can shed their outer hydrogen layers before they die," says Smith. "If Eta Carinae is able to shed 10 solar masses every thousand years or so, that's an efficient mechanism for peeling off a large fraction of the star."

Much fainter than a supernova, the explosion that generated the
fast-moving blast wave around Eta Carinae would likely have been similar to faint stellar explosions, sometimes called "supernova imposters," now being discovered in other galaxies by Earth-based robotic telescopes and other supernova searches. Looking at these galaxies, astronomers have seen stars like Eta Carinae that get brighter, almost approaching the brightness of a typical supernova.

"We don't know what they are,” says Smith “It's an enduring mystery as to what can brighten a star that much without destroying it completely."

Regardless of whether supernova impostors are scaled-down versions of supernovae, failed supernovae, precursor events or entirely different kinds of explosions altogether, studying them will yield important clues for understanding the last violent phases in the lives of massive stars.

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